Forest regeneration may not be able to keep up with global warming
Science guide efforts to make forests more resilient
Perched near a silvery, fallen log log at the edge of a Rocky Mountain forest clearing, warblers and nuthatches peck at the cones from nearby Engelmann spruce trees, but there aren’t many to be found. Since the mid-1970s, Colorado’s iconic conifers have been struggling to reproduce, mirroring a widespread global decline of forest ecosystems.
Because global warming is making this part of the Rockies warmer and drier, the handsome blue-green trees could soon face a regeneration bottleneck, says University of Colorado, Boulder forest researcher Robbie Andrus, describing the findings of a recent tree-ring research project a couple of miles high in the mountains west of Denver and Boulder.
“Reseeding events were widespread before 1975, but In recent years there has only been one or so establishment event,” he said. The repeated events came during a cooler and moister regional climate regime going back at least to the 1920s, based on other tree ring studies, as well as river flow measurements. Recruitment declines will continue as global warming drives increased moisture deficits.
“Establishment events have fluctuated in the past, but what’s disconcerting about the present situation is that we’ve seen increases in tree mortality at the same time. The same conditions increasing mortality are also unfavorable to establishment events,” he said.
The painstaking analysis of more than 150,000 tree rings identified the exact birth year of 450 Engelmann spruce and 500 subalpine firs. By matching those dates against climate and snowpack data back to 1940, the scientists found that, since 1975, seedling establishment has decline significantly.
The results offer more proof that global warming is throwing the delicate forest-climate cycle out of whack. Hotter and drier conditions correspond to significantly lower seedling densities and increased regeneration failure. That suggests that climate change could reduce forest density on a large scale.
Not just the Rockies
Scientists who took a broader look at 1,485 forest sites from Colorado to Washington state and reached similar conclusions — hotter and drier conditions correspond to significantly lower seedling densities and increased regeneration failure. That suggests that climate change could reduce forest density on a large scale, according to their study, published in Dec. 2017 in the journal Ecology.
Such findings can’t be extrapolated globally because there are so many different types of forest systems and there haven’t been detailed regeneration studies in all of them. Some forests, notably in the Pacific Northwest, are seeing an increase in regeneration.
It’s important to track such changes because changes on a global scale have impacts beyond local and regional ecosystems. Tom Evans, a London-based forest scientist with the Wildlife Conservation Society, says the growing forest malaise has feedback effect on climate. As forests decline, they will take less CO2 out of the atmosphere, which means more warming.
Evans says it’s important to focus on re-establishment events in addition to the well-documented die-offs, because it would show whether forests will grow back after wildfires or insect outbreaks.
“A minority of the world’s forests are truly intact, and we shouldn’t make a presumption they never will be damaged. Those areas are at risk and we should be mindful,” he said. With careful monitoring, some aspects of damage can be reversed. “You can get fire regimes back to them, and eliminate fragmentation.”
In a paper published Feb. 26 in the journal Nature Ecology & Evolution, Evans and other scientists suggested that the cumulative impacts of gradual forest degradation will be significant. “The point of our paper is to raise a flag. Forest cover is important but we shouldn’t concentrate only on forest loss,” he said.
Global warming is here, now
The sharp downward trend in big reproduction events in the Rockies — along with beetle epidemics and aspen-die-offs — is another warning sign that climate warming is already taking a toll on forest health in some areas. Global warming impacts are not a far-off scenario. They are already happening right now, said CU-Boulder forest expert Tom Veblen, who worked on both studies.
“Climate warming is, and has been, significantly and adversely affecting ecological processes in forests of the southern Rockies,” he said.
On Earth’s present global warming path, spruce and fir seedling establishment will continue to decline in coming years, and understanding that change can help forest managers adapt, said Andrus.
Spruce and fir forests define a big swath of the Colorado High Country, with rich habitat for wildlife, including the country’s largest elk herds, and the elusive and threatened lynx. The forests are also key watersheds for major western rivers, including the Colorado, the South Platte and the Rio Grande, and changes in forest cover can amplify other global warming impacts to the snowpack.
If resource managers want to try and sustain sustain spruce-fir forests, they need to adapt to the changing conditions, a tricky challenge, according to Andrus.
“You have to try and pick the appropriate years. We have a pretty good idea in April if there’s a good snowpack,” he said, explaining that more accurate season weather and climate projections will help identify the best times to try replanting, if needed, or to monitor whether other species of trees will start to replace the spruce and fir.
There’s a growing body of research showing how hotter drought is proving increasingly lethal for forests, says Grant Elliott, a forest geographer at the University of Missouri who has done related research.
The new study is critical for understanding how the structure and extent of mountain forest belts will change under future climate change scenarios, “ almost certainly involving the expansion of hotter drought,” Elliott said.
“I also think it’s interesting that site-level differences were not significant, meaning that trees on dry south-facing slopes responded similarly to trees on relatively wet north-facing slopes,” he said, adding that some of his own research shows a more topographically nuanced response to changing conditions. The fact that spruce and fir responded to climate change similarly could portend broader consequences for forests, he added.
